Glaze firing schedule for gas kiln

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I'm new to the forum...I kind of nosed around and didn't see anything that could answer my question. Everyone seems so knowledgeable and friendly, so I'm diving right in.

I have been put in charge of a school's ceramics program with an old tried-and-true Amaco gas kiln (9 cubic feet) for glaze firing. The previous teacher didn't leave any info or logs behind, so I've had to ask around to see what is best for glaze firing in gas kilns (but everyone seems to have electric kilns at schools these days!). I've been doing fairly well with glaze firings but it's been taking from 7 am to 5 pm to reach ^6 temp, and I'm stuck there after school waiting and waiting...is this a normal time frame for a glaze firing? How long should these glaze firings take?

Can anyone out there give me a ballpark estimate as to what the temperature ramp over the length of the firing should be? I would appreciate it!

Thanks!

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Do you pilot the kiln overnight? Does the kiln have a limit timer or any kind of computer control? When you fire it is it even top to bottom or is one hotter than the other. I ask these questions so advice can be tailored to best meet your needs.

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10 hours is probably too short for a ^6 glaze fire. We never do less than 12 and our reference is John Britt. We fire no faster than 100C/per hour until it reaches 600C. Then we can go 125 to 150 pretty easily. The first hour or so of firing is done with the door open and one burner until we reach the first 100C. They we may close it loosely to keep the heat rising no faster that 100C/hr. After 200 we shut it tightly and fire slowly until 600 then we light the 3rd and 4th burners. We usually only light the 2nd burner after 250 or 300C is reached.

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If everything in the kiln has been bisque fired, there's no reason to go slowly at the beginning. Give a preheat to make sure everything is dry, and then feel free to go fast. You won't blow up any pots. There is no benefit to the clay and glazes in firing slowly at the beginning. My gas schedule was about 4 hours to cone 08, stall it out for 45 minutes during body reduction, then lighten up on the reduction and climb to cone 11, then about 10 minutes of heavy oxidation before shutting it down. Total firing time about 9 hours. You should be able to get your down to 8-9 hours to cone 6 and still have great looking glazes.

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If everything in the kiln has been bisque fired, there's no reason to go slowly at the beginning. Give a preheat to make sure everything is dry, and then feel free to go fast. You won't blow up any pots. There is no benefit to the clay and glazes in firing slowly at the beginning. My gas schedule was about 4 hours to cone 08, stall it out for 45 minutes during body reduction, then lighten up on the reduction and climb to cone 11, then about 10 minutes of heavy oxidation before shutting it down. Total firing time about 9 hours. You should be able to get your down to 8-9 hours to cone 6 and still have great looking glazes.

I wish to second that. As long as the pots are dry you can fire really fast in kilns with draft (like gas, oil, wood). I will give some examples from my own work but keep in mind that different firings gives different results.

With bisqued ware I used to fire my woodkiln to 1050 degrees celcius in two hours. At this point i reduced and reached cone 9 in two hours. On raw ware it is also possible to fire fast as long as it is dry (If unsure I use a small heater to keep the kiln at 50 degress during the night with the chimney open a little). In my oil fired kiln i regularly fire large raw ware pieces (handbuildt sculpures and large vases) to cone 10 in 5 hours (reduction of body). When accidents happen it is with thick walled pieces not dry (cracking early, ~200 degrees C) and with pieces already fired to stoneware temp (cracking before the kiln starts to glow).

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Whoo Hoo! I'm glad you guys confirmed my 'impatience' technique. Riorose had me feeling a little inadequate. I give it a little while to make sure everything's dry, then I'll push pretty hard.

The only thing is, if you're 'stuck with' a gas kiln and want to use oxidation glazes that don't 'like' reduction, you should be careful not to push too hard. My propane kiln is hard to keep out of reduction. Fortunately, I don't use any glazes that object, even though I'm not developing reduction glazes yet.

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It is a gas kiln, which is updraft. It's about 40 years old and a workhorse.

It has a Chromalox on it, but it doesn't shut off unless I shut it off...so I have to kiln sit!

When I turn it on, both burners go on; I can't operate them independently.

It has dampers, which I keep open approximately 3 inches.

I do oxidation firings only so far. Once I get the hang of firings I may dabble in reduction!

I've tried it with a slow ramp and without. I seem to reach ^6 around 5 pm (starting at 7) no matter what I do. I had some minor warping of pieces so I thought a slower ramp up would prevent that, but it still happens.

I'm very lucky that the glazes I'm using have done well in the firings (Mastering Cone 6 Glazes and some other random ones).

And when I say "stuck" I mean it; they built the kiln room around the two gas kilns we have! Those puppies are there for good!

thanks again!

Robyn

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Sounds like you have an old Alpine updraft. You shouldn't need to operate the burners independently, since they are manifolded to get equal pressure. Don't use the Chromalox for anything but a safety shutoff. Use cones and shut it off manually. And don't use the Chromalox for ramping the firing. You'll probably get uneven firings if you do. Fire everything manually!

If you're getting warping, it's probably not from the speed of the firing. It could be from the speed of the cooling, though, or from over-firing slightly. Cool slowly and really watch the cones.

You should be able to speed up the firing considerably, assuming you've got enough gas pressure. Where the main line meets the burner system there should be a gas pressure gauge. It should read at least 12 IWC, preferably 14 or more.

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My updraft propane 24 cu' kiln actually fires faster if the damper is pushed shut enough to develop a little backpressure. How can you tell? When you pull out an upper peep plug and a huge flame shoots out, the damper is too far SHUT. If you pull out the upper peep plug and there is nothing coming out of the view port, your damper is too far open. Just adjust the damper (along with the amount of gas being used) so that a small licky flame comes out the top peep. This is generally regarded as a neutral atmosphere, and at least in my kiln, this setting gives the highest rate of climb.

Of course when you want a body reduction, you simply close the damper as necessary. Same for the final glaze soak. BTW, when I reduce, I abhor great billowing plumes of smokw. I know, I know, lots of potters feel this is needed to get a good reduction. Excuse me, but I have been building and firing kilns since 1970, and I think I know what I am doing.

Please take this in the spirit it is being given--solely to help. Best of luck to you.

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I agree with the firing quick if all is bisques. I don't know the size kiln you are talking about but I fired large gas downdraft car kiln about 60 cu. ft. of my design for man ears and recently an old Olsen updraft about 64 cu. ft. both to ^6. I would say 6-10 hours depending on all the givens...stacking density , duration of soaks, weather, etc. I body reduce from ^09 to 04. I reduce the glazes after ^5 is going. This last firing I had a slight reduction most of the way up. Got some great shino carbon trapping. I started at 7:30 and finished around 4.

Marcia

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I picked up this oldthread as it seemed the most appropriate place to put this question. We justfinished building a new studio and did our first glaze firing in a used (80 orso firings) 24 cu ft West Coast updraft using propane.

I have fired numerous downdraft kilns from 10 to 95 cu ft but this was my firstupdraft. My question relates to being able to see the cones at the bottom peep.I literally could not see them unless I turned the kiln way down so there waslittle turbulence from the burner and put the kiln in oxidiation. I believe the issue is the burner is firingright in front of where the cones are set - who dreamed that up? The top conesare no problem.

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You don't want to put anything directly in front of the burner port. Assuming we're talking about burners going in horizontally, there should be a target brick in the middle of the firebox that breaks up the flame.

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^You folks are so lucky to have this knowledge and understanding of gas kilns and reduction. I want so badly to own a gas kiln but can't afford one and am afraid it would be too difficult to fire. I am humbled by the knowledge you guys have Do used gas kilns ever appear for sale? I have never seen on advertised...but who would want to give one up.

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You don't need a gas kiln! I fired cone 10 reduction for 15 years, then switched to cone 6 electric and never looked back. I'm making the best pots and glazes I ever have. Don't feel like gas is better than electric. It's not. It's just different.

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You don't want to put anything directly in front of the burner port. Assuming we're talking about burners going in horizontally, there should be a target brick in the middle of the firebox that breaks up the flame.

Actually the burners are vertical - coming up through the floor - right in front of the bottom peep. The idea is to put a post or refractory across the middle of the port so that you are not blocking it - just moving the flame to each side.

I also heard today that a local teacher user a rebar pole and he touches the cone when he can't see it. This seems to make the cone easier to see. I am going to try that next time.

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My little 12 cubic Burman updraft has 6 burners pointing up like yours-(Its almost to cone 10 now) and I do not even put cones on bottom as its always hotter there and I fire it only with top cones.I load all the need heat glazes on bottom and cool glazes on top and its worked great like this for past 20-25 years.I know its always a cone or more off on bottom and just accept it.